The present invention relates to GNSS (Global Navigation Satellite System) navigation and, more particularly, to GNSS navigation assisted by static data.
A GNSS system is a space-based global navigation system in which the satellites of a constellation of satellites in orbit around the Earth transmit signals that are used by terrestrial receivers for purposes such as navigation. Such signals are called “ranging signals” herein. The existing GNSS systems include the GPS system operated by the United States of America and the GLONASS system operated by Russia. Forthcoming GNSS systems include the European Galileo system and the Chinese Compass navigation system.
The operation of the GPS system now will be described as exemplary of how GNSS systems in general operate. The GPS system includes between 24 and 32 satellites in medium Earth orbit. The GPS ranging code that is available for public use is the C/A code which is a PseudoRandom binary Code (PRN) of 1,023 bits. Each satellite continuously transmits its own unique PRN code that is orthogonal to all the other PRN codes. Modulated on top of each satellite's C/A code is a navigation message that includes an ephemeris (plural: ephemerides) that describes the satellite's orbit. The ephemerides are updated every two hours.
A GPS receiver generates its own copies of the C/A codes, nominally synchronized with the satellite transmissions. When the receiver receives the C/A codes of the currently visible satellites (typically 12-16 satellites in open terrain), the receiver cross-correlates its copies of the C/A code with the received C/A codes to determine the one-way travel times from the satellites to the receiver. Knowing the ephemerides, the receiver computes the locations of the satellites at the times of the transmissions to obtain ranges to the satellites. If the receiver's clock were perfectly synchronized with the satellites then signals from three satellites would suffice to triangulate the position (horizontal coordinates x and y and elevation z) of the receiver. Because clocks that could be synchronized that well with the atomic clocks used by the satellites are far too expensive for routine use, signals from four or more satellites are used to determine x, y, z and the time offset dt of the receiver's clock from the GPS clocks. If signals from only three satellites are available for navigation, it is possible to use an estimate of z for calculating x, y and dt. The accuracy of the calculated x and y depends on the accuracy of the estimate of z.
The use of external data resources is a common practice in modern GNSS receivers. External data sources typically include a network server providing data that helps a satellite navigation receiver to achieve better performance. By using external aiding sources, a satellite navigation receiver may achieve improved sensitivity or shorter time to first fix (TTFF). Aiding data may include the user's approximate location, current time and satellite navigation messages that contain ephemeris and almanac data.